1. Field of the Invention
The present invention relates to a nitride semiconductor laser.
2. Description of the Related Art
A surface-emitting laser, which is one type of semiconductor laser, is characterized by emitting light in a direction perpendicular to a substrate. A vertical-cavity surface-emitting laser, which includes an active layer between distributed Bragg reflectors, is practically used at infrared wavelengths. However, in a GaN-based vertical-cavity surface-emitting laser having a lasing wavelength in the ultraviolet to green wavelength region, it is difficult to manufacture distributed Bragg reflectors due to the physical properties of nitride semiconductors. In particular, it is very difficult to develop a vertical-cavity surface-emitting laser capable of current injection in the wavelength region. Thus, a distributed feedback (DFB) surface-emitting laser containing two-dimensional photonic crystals has recently been investigated. The distributed feedback surface-emitting laser containing two-dimensional photonic crystals is hereinafter referred to as a DFB surface-emitting laser.
Two-dimensional photonic crystals of a DFB surface-emitting laser function as a second-order diffraction grating for the lasing wavelength. Thus, a feedback effect (in particular, an amplification effect in a gain region) can be achieved as second-order diffraction, and a laser beam is emitted in a direction perpendicular to a substrate by a first-order diffraction.
FIG. 7 is a cross-sectional view of a DFB surface-emitting laser 1100 according to Japanese Patent Laid-Open No. 2006-165255 (FIG. 2, paragraph [0034]).
The DFB surface-emitting laser 1100 includes an n-type cladding layer 1108, an active layer 1104, and a p-type conductive layer 1111 on an n-type substrate 1107 in this order. The active layer 1104 includes a well sublayer 1112 and barrier sublayers 1113. The p-type conductive layer 1111 includes a minority carrier blocking sublayer 1109, a two-dimensional photonic crystal sublayer 1101, a p-type cladding sublayer 1102, and a semiconductor sublayer 1103. The DFB surface-emitting laser 1100 further includes a p-electrode 1105 and an n-electrode 1110 at the top and bottom thereof.
In the surface-emitting laser 1100, the two-dimensional photonic crystal sublayer 1101 is composed of a high-refractive-index medium 1001 and a low-refractive-index medium 1002. The high-refractive-index medium 1001 is p-type GaN and has a thickness of 100 nm.
The p-type cladding sublayer 1102 is formed of p-type AlGaN and has a thickness of 500 nm. The thickness of the p-type cladding sublayer 1102 is determined such that light produced in the well sublayer 1112 is attenuated in the p-type cladding sublayer 1102 to such an extent that the optical absorption loss in the p-electrode 1105 is sufficiently small. It is therefore desirable that the p-type cladding sublayer 1102 have a sufficient thickness with respect to optical confinement.
As described in Japanese Patent Laid-Open No. 2006-165255, in a nitride semiconductor laser having a separate-confinement heterostructure, p-type AlGaN is generally used as a material of a p-side cladding layer.
However, because the p-type nitride semiconductor has an optical absorption loss resulting from a Mg dopant, it is desirable that the thickness of the p-type conductive layer be reduced. In particular, since p-type AlGaN has a higher electrical resistance than the same p-type conductive, p-type GaN, a further improvement is required to decrease the lasing threshold.
In order to decrease the lasing threshold, the thickness of a p-type AlGaN layer may be decreased, or the Al content in a p-type AlGaN layer may be decreased to reduce the resistivity of the p-type AlGaN layer.
However, a p-type AlGaN layer having a small thickness has an insufficient function as a cladding layer and cannot provide a sufficient thickness to reduce light leakage to the p-electrode. This results in a large optical absorption loss in the p-electrode, leading to poor laser performance.
A decrease in Al content in a p-type AlGaN layer to reduce the resistivity of the p-type AlGaN layer results in an increase in the refractive index of the AlGaN layer, thereby increasing light leakage toward the p-electrode. This also results in an increase in optical absorption loss in the p-electrode, leading to poor laser performance.